1. Field of the Invention
The present invention relates to a fabrication method, particularly to a method for smoothing a surface of an optical element with a laser.
2. Description of the Related Art
In optoelectronics, the optical resonator is a very important element, which enhances the intensity of light having a specified wavelength and has been widely used in various optoelectronic devices, such as micro-laser devices, optical filters, optical switches, ultra-fine sensors, high-resolution spectrometers and so on.
Based on the advanced semiconductor technologies, many researches are devoted to tiny and low-cost integrated optical elements. However, the surface roughness of a tiny optical resonator results in too high a scattering loss and impairs the realization of a tiny and high-density optical integrated circuit.
J. S. Xia et al. fabricated a disc-type optical microcavity and observed therefrom a silicon emission spectrum intensified by 30 times at a room temperature and a whispering gallery mode. The optical resonator has a Q value (quality factor) of 500-700, and they believe that the Q value can be greatly increased by smoothing the roughness of the lateral surface. Satoshi Iwamoto et al. fabricated a photonic-crystal submicron optical resonator and observed a photoluminescence enhanced by 310 times therefrom. This opticalresonator has a Q value of about 5000. However, the observed Q value is much smaller than the theoretical Q value 21000, and they suppose that the fact results from the lateral surface roughness generated in fabrication.
Many methods have been used to improve surface roughness and decrease scattering loss, including the hydrogen annealing method, the dry oxidation method, and the wet chemical etching method. In the hydrogen annealing method, a sample is placed in a hydrogen-filled furnace at a temperature of 1100° C. for 3 minutes, whereby the RMS (Root-Mean-Square) roughness is reduced to 0.11 nm. In the dry oxidation method, a sample is placed in a furnace at a temperature of 1100° C. for 50 minutes to form an autogenous oxide layer, whereby the RMS roughness is reduced to 0.5 nm. In the wet chemical etching method, the surface of a sample is oxidized with a chemical solution, whereby the RMS roughness is reduced to 0.7 nm.
The abovementioned methods respectively have their advantages and disadvantages. In the hydrogen annealing method and the dry oxidation method, the sample has to suffer a high temperature. In general, such a high temperature will seriously damage the crystalline quality of the electronic circuit. The dry oxidation method and the wet chemical etching method can only improve surface roughness to a limited extent. The wet chemical etching method not only consumes the material but also varies the topography of the material.
Accordingly, the present invention proposes a method for smoothing a surface of an optical element with a laser to solve the abovementioned problems and improve the resonator and waveguide performance of an optical element.